The present invention generally relates to the production of purified sugar beet juice, and more particularly to a highly efficient purification process for beet juice which uses recycled lime and recycled calcium carbonate compositions. As a result, waste materials and reaction by-products are substantially eliminated while greatly improving the efficiency of the entire treatment process.
Sugar beets provide a substantial and economical supply of sugar (sucrose) for many purposes. To obtain a completed, purified sugar product from sugar beet materials, raw beets are treated to remove juices therefrom. The juices are thereafter subjected to numerous isolation and purification steps so that a final sugar product can be obtained. In a typical treatment process, sugar beet juice is obtained by physically slicing raw beets into strips or "cossettes" which are placed in contact with heated water. As a result, sugar-containing beet juice is extracted to produce a raw juice product. This material normally contains a moderate amount of solid particulate matter therein (primarily derived from the initial raw beets). In most cases, the raw juice product contains about 0.2-0.5% by weight solid, undissolved materials, and about 13-16% by weight water-soluble compositions.
The raw juice is thereafter treated using one or more mechanical screening processes to remove most of the solid matter therefrom. The resulting strained juice product is commonly known as "diffusion juice". This material (which is ultimately subjected to additional treatment steps) typically includes water, sugar (sucrose), dissolved non-sugar impurities, and colloidal (suspended) non-sugar impurities. The non-sugar impurities are often highly colorized and/or thermally unstable. In this regard, they can interfere with subsequent treatment steps and adversely impact the quality and quantity of the final sugar product. It is therefore necessary to remove as many non-sugar impurities from the diffusion juice as possible. At this stage in the process, the diffusion juice typically contains about 82-85% by weight water, about 13-15% by weight sugar, and about 2.0-3.0% by weight dissolved/colloidal non-sugar impurities. The overall weight of the diffusion juice is about 105-130% of the weight of the raw beet materials being processed.
Conventional beet juice purification processes initially involve a step known as "preliming" in which the pH of the incoming juice materials is raised from an initial (starting) level of about 6.3-6.5 to a level above about 11.6 (e.g. optimally between about 11.5-11.8). Preliming of the diffusion juice provides many functional benefits, including improved flocculation of impurities (discussed below), as well as enhanced pH stabilization, decolorization, and filterability. In particular, preliming enables substantial amounts of undesirable materials to be precipitated from the juice. These materials include but are not limited to inorganic acids, organic acids, phosphates, metal ions (e.g. iron, aluminum, and/or magnesium ions), proteins, pectins, coloring agents, and saponins.
The addition of lime materials to accomplish preliming is typically undertaken in a preliming vessel. While many different vessels can be used for this purpose, a preferred system is disclosed in U.S. Pat. No. 2,774,693 to Brieghel-Meller which is incorporated herein by reference and further discussed below. The particular system disclosed in the foregoing patent involves a tank having a plurality of chambers and multiple mixing blades therein. Within the tank, lime (and calcium carbonate materials) are added to the incoming diffusion juice in a controlled, gradual, and progressive manner, with the particular details of this process being discussed in U.S. Pat. No. 2,774,693 cited above. The terms "lime", "lime materials", "milk of lime", and "lime compositions" as used herein shall be considered equivalent, and will specifically involve calcium oxides or calcium hydroxides in solid or liquid (suspension) form. Milk of lime is preferred in most juice purification processes, and consists of a suspension of calcium hydroxide (Ca(OH).sub.2) which is formed in accordance with the following reaction: EQU CaO+H.sub.2 O.revreaction.Ca(OH).sub.2 +15.5 Cal.(1)
Within the preliming tank/containment vessel, a highly efficient treatment process occurs in which the pH of the diffusion juice materials is increased as noted above. This procedure causes non-sugar materials (especially proteins in both dissolved and colloidal form) to flocculate and produce a plurality of solid floc particles. Regarding non-sugar proteins, gradual pH increases which result from the preliming process enable these materials to reach their respective isoelectric points. The term "iso-electric point" as used herein shall involve a pH level at which protein colloidal particles within the diffusion juice materials have a zero electrical potential. When the foregoing materials reach their designated iso-electric points, they flocculate to form a plurality of solid floc particles as indicated above. Flocculation is further enhanced by the addition of calcium carbonate materials which functionally form a core or substrate which is surrounded (encapsulated) by the flocculated protein materials. This process increases the weight and density of the floc particles, thereby facilitating the filtration/settling of such materials and removal from the system.
In conventional processing systems, the resulting mixture of diffusion juice, residual lime, excess calcium carbonate, and floc particles is subjected to a plurality of subsequent steps which are performed in a sequential manner. Specifically, the mixture is first subjected to a cold main liming stage undertaken at a temperature of about 30.degree.-40.degree. C. in which additional amounts of lime (milk of lime) are added, followed by a hot main liming stage performed at a temperature of about 80.degree.-90.degree. C. which involves the further addition of lime (milk of lime). These steps cause the pH of the limed product to increase to a level above about 12.6. As a result, dissolved/colloidal non-sugar materials which were not affected during preliming of the diffusion juice are decomposed. In particular, main liming of the juice materials as described above achieves juice thermostability by partial decomposition of invert sugar amino acids, amides, and other dissolved non-sugar) materials. If not eliminated, these materials can cause serious problems in subsequent processing steps when the juice product is exposed to high temperature and pressure levels.
After main liming, the treated juice product is subjected to a first carbonation stage in which carbon dioxide gas is combined with the product. The carbon dioxide gas reacts with residual lime in the treated juice to produce a substantial amount of calcium carbonate in the form of a fine precipitate. Not only is residual lime removed using this procedure (e.g. typically about 95% by weight of the residual lime), but the surface-active calcium carbonate precipitate is able to adsorb substantial amounts of remaining dissolved non-sugar contaminants. Furthermore, the calcium carbonate precipitate functions as a filter aid in the physical removal of solid materials from the treated juice (e.g. the removal of floc particles). In this regard, the diffusion juice is efficiently filtered and further purified.
The purified juice product is then subjected in most cases to additional heating, one or more additional carbonation steps, and further filtering to produce a purified product conventionally known as "thin juice" which is ready for evaporative thickening and sugar crystallization. Further information regarding this process and related procedures used to treat/purify sugar beet juice materials is provided in U.S. Pat. Nos. 1,578,463 to Nicholson et al.; 1,815,276 to Schweiger; 2,164,186 to Brown et al.; 2,547,298 to Wiklund; 2,557,800 to Seailles; 2,697,049 to Brieghel-Muller; 2,774,693 to Brieghel-Muller; 2,824,028 to Zenzes; 2,977,253 to Grandadam; 3,089,789 to Van Note; 3,113,044 to Alston; 3,168,419 to Gale; 3,734,773 to Haley; and 5,320,681 to Moc et al. which are all incorporated herein by reference.
Notwithstanding the ability of the foregoing process to purify sugar beet juices, it has been discovered that certain floc particles produced during the preliming stage are sensitive. These materials can be destroyed (e.g. resolubilized) by the harsh mechanical/chemical conditions which are present within the main liming and carbonation stages of the process. In most cases, destruction of these materials is caused by the repeptization of protein compositions within the floc particles, as well as physical damage to the floc particles caused by mechanical agitation within the system. As a result, contaminants are reintroduced into the juice materials. To avoid these problems, a processing system was developed that is disclosed in co-owned U.S. Pat. No. 4,795,494 to Toth et al. which is incorporated herein by reference. To avoid reintroduction/dissolution of previously-formed floc materials, U.S. Pat. No. 4,795,494 discloses a method in which the product from the preliming stage (consisting of juice materials combined with floc particles and other dissolved/colloidal contaminants) is temporarily isolated from the system. Instead of directing this product into the main liming stages of the system, it is transferred into a separation system (e.g. a gravity settler tank or comparable unit) in which the floc particles are separated from the prelimed juice fraction. The prelimed juice fraction is then routed into the main liming stages of the process and subsequently treated in one or more carbonation chambers as discussed above. However, the removed floc particles (and residual amounts of juice materials associated therewith) are routed past the main liming stages of the system. These materials are instead routed into the carbonation stages of the process where they join with the initial juice fraction after it has passed through the main liming stages. Because the floc materials are not subjected to the harsh physical/chemical conditions within the main liming stages of the process, the floc particles do not re-dissolve and are readily removed by settling and filtration after carbonation is completed. Not only does this method avoid the re-introduction of contaminants into the purified juice, but likewise ensures that any residual juice materials associated with the floc particles are collected in subsequent portions of the process to obtain maximum sugar production. Further details of this process and variations thereof are again discussed in U.S. Pat. No. 4,795,494 to Toth et al.
Notwithstanding the benefits described above, the foregoing methods (and other, more conventional beet processing systems) generate substantial amounts of calcium carbonate waste products. These products primarily involve calcium carbonate sludge materials obtained from the carbonation of limed juice fractions. They are normally collected and stored in large, open areas, thereby presenting problems from both a safety, environmental, and space-conservation standpoint. The present invention involves a unique and highly efficient method for reclaiming/recycling calcium carbonate waste products so that these materials can be efficiently reused in selected multiple portions of the processing system. As a result, the problems associated with calcium carbonate waste materials are substantially eliminated in a manner which not only prevents environmental problems, but actually improves the efficiency of the entire treatment process by significantly reducing material costs. Accordingly, the multi-stage recycling process of the present invention represents an advance in the art of sugar beet processing, sugar production, and the elimination of waste products as discussed below.